pkg(8), via its daily periodic script, requires xz. We don't have
a way to encode dependencies from ports on base packages right now,
so instead move xz to the minimal set so it's always installed.
This isn't an ideal solution, but it's justified in this case since
pkg(8) is always installed, so having its dependencies always
installed is acceptable.
MFC after: 1 week (stable/15 only)
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